Charles and Titus presented a summary of a large body of work that has been done with the Avida system at CalTech. The Avida system is a derivative of the Tierra system, with perhaps the biggest difference being that the digital organisms are mapped into a two dimensional grid, where one creature occupies each cell of the grid, and the genomes of the creatures exist in a space orthogonal to the 2D grid.
The Avida group has been using Avida to study the physics of evolution. Tom expressed the opinion (reiterated here) that the work of the Avida group is the most beautiful work done with a Tierra-like system. For those not at the workshop, look to the following publications for more information:
C. Adami, ``Learning and Complexity in Genetic Auto-Adaptive Systems'' Physica D 80 (1995)154.
C. Adami, ``Self-Organized Criticality in Living Systems'' Caltech preprint 1994, submitted to Phys. Lett. A
C. Adami, ``On Modeling Life'', Artificial Life 1 (1994)428; Proc. ``Artificial Life IV'', (MIT Press) 1994, p. 269
C. Adami and C.T. Brown, ``Evolutionary Learning in the 2D Artificial Life System `Avida' '', Proc. ``Artificial Life IV'', (MIT Press) 1994, p. 377
C. Adami, C.T. Brown, and M. Haggerty, ``Abundance Distributions in Artificial Life and Stochastic Models: "Age and Area" revisited''; 3rd European Conference on Artificial Life, June 4-6, 1995, Grenada, Spain (to appear)
C. Adami and C. Ofria, ``Speciation in Single-Stranded Self-Replicating Code'' Caltech preprint (1995)